The present invention relates generally to inertial guidance systems and, more particularly, to an inductively coupled force balance instrument for measuring acceleration in such systems.
Commercially available force balance instruments often have a sensitive element mounted mechanically to a base by means of a flexure suspension. Unfortunately, flexure suspensions can introduce extraneous forces which contribute to bias error. Mechanically suspended systems also permit vibropendulous rectification associated with aniosoelastic compliance of the sensing element. Such considerations make it difficult to meet existing design requirements for strap down inertial navigators and accelerometers.
Prior force balance systems also suffer in accuracy when a coil or a permanent magnet is mounted to the sensitive element. Coils on the sensitive element are typically connected to a current or voltage feedback source by flexible leads which can introduce error forces and resonant frequencies. The leads must be insulated from each other and from the sensing element, causing further instability and hysteresis.
Magnetic material on the sensitive element can cause error forces due to interaction with the case, variations in external fields and permeability of surrounding structures. The size, weight and inertia of a permanent magnet and its high permeability circuit present additional design problems, including high power requirements and consequent heat dissipation.
One method of improving bias repeatability and stability and reducing rectification error in a force balance instrument is to eliminate mechanical and electrical connections to the sensitive element. This has been done by supporting a sensing element electrostatically or electromagnetically in three axes. Systems of the electrostatic type, such as that described in A. Barnard, et al., "Three-Axis Electrostatic Accelerometer", Symposium Gyro Technology (1985), pp 12.0-12.23, exhibit square law forcing which is undesirable in force balance instruments. Prior electromagnetically supported systems, on the other hand, have high permeability sensitive elements which suffer from the disadvantages described above for systems with permanent magnets mounted to the sensitive elements. They can also introduce undesirable sensitivities and variations in magnetic properties as a function of angular position due to inhomogeneities in the magnetic material.
Therefore, it is desirable in many applications to provide a force balance instrument having improved bias stability and reduced rectification error while providing an output proportional to a sensed input. It is also desirable to provide a three-axis instrument which relies on linear forcing and has a sensing element of low mass and low permeability.